This two-part study will evaluate how safe LY3154207 is and the effects it has on the body.

Part A will include healthy participants. Each participant will receive daily doses of LY3154207 or placebo for 14 days. Part A will last approximately 4 weeks including a 17 day stay in the clinical research unit (CRU) and follow-up.

Part B is contingent on the results of Part A. Part B will include participants with Parkinson's disease. Each participant will receive daily doses of LY3154207 or placebo for 14 days. Part B will last approximately 4 weeks including a 17 day stay in the CRU and follow-up.

Both Part A and Part B will require screening within 30 days prior to the start of the study.

Inclusion Criteria:

Part A:

• Overtly healthy males or females, as determined by medical history and physical examination
• Female participants not of child-bearing potential

Part B:

• Have a clinical diagnosis of idiopathic Parkinson's disease for at least 1 year and on stable medication for at least 4 weeks

Part A and B

• Are reliable and willing to make themselves available for the duration of the study and are willing to follow study procedures
• Have given written informed consent
• Have a body mass index (BMI) of 18.0 to 29.9 kilograms per square meter (kg/m²)

Exclusion Criteria:

• Have participated, in the last 30 days, in a clinical trial involving an investigational product
• Have a significant history of or current cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrine, hematological or neurological disorders capable of significantly altering the absorption, metabolism or elimination of drugs; or constituting a risk when taking the study medication; or interfering with the interpretation of study data